I find the Brodit Proclip system to be the best. 6 and 6+ mounts should be available by end of October
I bought the Universal Adjustable Brodit/Proclip mount 511479 for my 6 Plus. There is a tiny gap depth wise as my cheap gel case is very thin but I think it will fit perfectly when my nice leather case arrives.
For those wishing to have their phone away from the windscreen, there are usually several options available for most cars, I have attached the options that were available for my car.